Defines the number of extra records to display when virtualizing

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    Number, default = 15
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Virtualization is the process when only records in the current scroll window are added to the actual DOM tree. This approach
allows grid to have millions of records and still be snappy. So, if grid has move the 150 records, then in the DOM there will be only
records that are in the view, plus 15 on top and bottom for better scroll performance.

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You should not normally change this property.